The long-term objective of this project is to develop small molecule inhibitors of antigen presentation for the treatment of multiple sclerosis (MS). MHC class II molecules are thought to play a critical role in the disease process by presenting peptides from myelin antigens to CD4 T cells. Genome-wide studies have demonstrated linkage to the MHC class II region, and the HLA-DR2 haplotype (DRB1*1501) is associated with disease susceptibility in both familial and sporadic cases. The investigator has previously determined the crystal structure of HLA-DR2 (DRA, DRB1*1501), which demonstrates features of the binding site that represent potential targets for specific inhibitors. Studies in mice deficient in DM or invariant chain have demonstrated that antigen presentation in the CNS is strictly required for the development of CNS inflammation. The major hypothesis of this project is that small molecules which block critical steps in the presentation of myelin antigens to CD4 T cells can prevent the development of MS lesions. The project will be performed in close collaboration with the recently established Harvard Center for Neurodegeneration and Repair (HCNR) that is developing a drug discovery program directed at neurological diseases. The goal of the center, and of this particular project, is to develop therapeutic approaches based on ongoing research efforts, and to test compounds in relevant animal models. The identification of inhibitors will focus on the mechanisms of peptide loading onto MHC class II molecules in antigen presenting cells. Particularly relevant is the DM catalyzed dissociation of the invariant chain derived CLIP peptide since it precedes binding of antigenic peptides to MHC class II molecules. The investigator has developed a mammalian expression system that provides suitable quantities of the HLA-DR2/CLIP complex for analysis of large and diverse libraries of small molecules. Preliminary studies demonstrate that fluorescence polarization provides a sensitive, real-time readout of peptide binding to these HLA-DR2/CLIP complexes. The specific aims of the project are: To develop assays for the identification of small molecules that block presentation of myelin peptides (Aim 1), to examine a large and diverse collection of small molecules under conditions relevant for intracellular peptide loading (Aim 2), and to study the mechanisms by which compounds inhibit presentation of myelin antigens (Aim 3).